The present invention relates generally to devices for heating batteries. More particularly, the invention relates to electrical circuits for using the stored energy of a battery to heat either the connected battery or a peripheral battery. The invention also relates to electrical circuits for transferring energy between a plurality of series connected batteries and equalizing the energy within the batteries.
With the current growth in electrical and electronic technology, there is a growing interest in using batteries as a primary source of power, for backup applications, and for starter, lighting, ignition (SLI) applications. Electric vehicles, hybrid vehicles, military electronic systems, consumer vehicles, communications systems, medical emergency equipment, and handheld power tools are among the more promising applications requiring batteries.
Batteries provide a uniquely portable source of energy that is not dependent on a connection to a power grid. However, the operation of a battery is limited by a number of factors, one of which is temperature. At low temperatures the capacity of a battery to store energy, the maximum value of discharge current which can be drawn from the battery, and the cold cranking amps capability decrease substantially. Many applications require batteries either as a primary power source, such as; electric vehicles, hybrid vehicles, and handheld power tools, or as; a backup power source such as communications systems, medical emergency equipment, and military electronic systems, or also as; an SLI source such as commercial vehicles. Batteries used for SLI applications at cold temperatures additionally must be capable of supplying greater amounts of cold cranking current to overcome the increased engine resistance caused by decreased engine oil viscosity at low temperatures. Most if not all of these devices are exposed to outdoor environments and therefor must be able to operate at low temperatures.
Current methods of operating batteries at low temperatures employ means for heating the battery from an external source, such as warm air heating, liquid heating, and thermal jackets. Conventional battery heating systems typically use a separate power source to power a heating element which generates the required heat. The resultant heat is then transferred to the battery by either a convection system or a conduction system. Convection systems blow hot air across the battery, whereas conduction systems apply heat directly to the surface of the battery. Each of these systems warm the battery by heating the external surface of the battery. By applying heat to the external surface a significant amount of the generated heat is lost to the external environment. Both convection and conduction systems additionally require some form of mechanical structure co-located with the battery. Also, warm air heating and liquid heating require complex mechanical systems that use substantial amounts of external power. These constraints limit the portability of batteries and demand the availability of an external power source.
A system employing thermal jackets typically includes a flexible insulator that wraps around the exterior of a battery. On the inner surface of the insulator is an externally powered heating element. While thermal jackets do not require a complex mechanical system, extra space around the individual batteries must be set aside. Additionally, the thermal jacket systems known in the art must be powered by a separate power source or power grid. While heating systems and thermal jackets can be used to extend the ambient temperature range within which batteries can be operated, they have not proven capable of efficiently heating a battery without an external power source and bulky external mechanical attachments.
Accordingly, it is desirable to overcome the disadvantages associated with the prior art systems. The present invention addresses this problem by circulating energy from within the battery into lossless or nearly lossless energy storage devices and then back into the battery causing the battery to dissipate power internally due to the electrical-to-chemical energy conversion losses and conduction losses of the battery. The internally dissipated power heats the battery, causing the internal temperature to rise. A higher internal temperature increases the operating temperature range, improves the cold cranking amps capability, and improves the energy capacity utilization of the battery. The circuit of the present invention does not require a complex mechanical structure, and efficiently heats the battery from the inside out. The circuit enhances the portability of batteries into cooler operating environments by using the internal energy of the battery for self-heating. The circuit takes energy from the battery and stores it in an essentially lossless element and then recirculates that energy back into the battery. In transferring energy first out of the battery and then back into the battery some losses will occur within the battery due to inefficiencies in converting electrical energy into chemical energy. The losses result in increased heat within the battery thereby causing the battery to heat from the inside out. Additionally, there are very few constraints on the location of the circuit relative to the battery. It is possible to place the circuit a significant distance away from the battery, depending on the magnitude of the circulating current. Additionally the circuit of the present invention allows the energy potential within the individual cells of an energy storage device to be equalized.